Recently, a magnetic random access memory (to be referred to as an MRAM hereinafter) using the magnetoresistance effect of a ferromagnetic material is awakening an increasing interest as a next-generation, solid-state nonvolatile memory having a high read/write speed and large capacity and capable of a low-power-consumption operation. In particular, a magnetoresistance effect element having a magnetic tunnel junction (to be referred to as an MTJ hereinafter) has attracted attention since the element was found to have a high magnetoresistance change ratio.
The MTJ basically has a three-layered structure including a storage layer having a variable magnetization direction, an insulator layer, and a reference layer maintaining a predetermined magnetization direction. When an electric current is supplied to this MTJ, the electric current flows by tunneling through the insulator layer. In this state, the resistance of the junction portion changes in accordance with the relative angle between the magnetization directions of the storage layer and reference layer. For example, the resistance takes a minimal value when the magnetization directions are parallel, and a maximal value when they are antiparallel. This resistance change is called a tunneling magneto-resistance effect (to be referred to as a TMR effect hereinafter). When using a magnetoresistance effect element having the MTJ as a memory cell, information is stored by making the parallel and antiparallel states of magnetization in the storage layer and reference layer (i.e., the minimal and maximal of the resistance) correspond to “0” and “1” of binary information. Note that the parallel and antiparallel states of magnetization may also be made to correspond to “1” and “0” of binary information.
As a method of writing information in the magnetoresistance effect element, a magnetic field writing method is known in which a write wiring is formed near a memory cell and only the magnetization direction in the storage layer is reversed by a current magnetic field generated when an electric current flows. When the element size is decreased in order to implement a large-capacity memory, however, the coercive force (Hc) of a magnetic material forming the storage layer increases in principle, so an electric current necessary for write increases as the element is downsized. On the other hand, the current magnetic field from the write wiring decreases in principle as the cell size decreases. In the magnetic field writing method, therefore, it is difficult to achieve both the downsizing of a cell required for large-capacity design and the reduction in write current.
On the other hand, as a write method of solving this problem, a writing (spin transfer torque writing) method using spin-momentum-transfer (SMT) has recently been proposed. This method reverses the magnetization direction in the storage layer by supplying a spin polarization current to the magnetoresistance effect element. In addition, the smaller the volume of a magnetic layer forming the storage layer, the smaller the amount of spin-polarized electrons to be injected. Accordingly, the method is expected as a writing method capable of achieving both the downsizing of an element and a low electric current.
When downsizing an element in order to achieve a large capacity, however, the energy barrier for maintaining the magnetization direction in the storage layer in one direction, i.e., the magnetic anisotropic energy becomes smaller than the thermal energy. Consequently, the problem that the magnetization direction of the magnetic material fluctuates (a thermal disturbance) and stored information cannot be maintained any longer becomes conspicuous.
The energy barrier required to reverse the magnetization direction is generally represented by the product of a magnetic anisotropy constant (the magnetic anisotropic energy per unit volume) and a magnetization reversal unit volume. To ensure a resistance to the thermal disturbance in a fine-element-size region, therefore, it is necessary to select a material having a large magnetic anisotropy constant. In-plane magnetization type arrangements mainly examined up to date generally use magnetic shape anisotropy. To increase the magnetic anisotropic energy in this case, it is necessary to, e.g., increase the aspect ratio of the magnetoresistance effect element, increase the film thickness of the storage layer, or increase the saturation magnetization of the storage layer. When the features of the spin transfer torque writing method are taken into consideration, however, all of these measures increase a reversal current and hence are unsuitable for downsizing.
On the other hand, it is also possible to use a material having high magnetocrystalline anisotropy instead of the magnetic shape anisotropy. In this case, however, the axis of easy magnetization in the in-plane direction largely disperses in the film surface. This decreases the MR ratio (Magnetoresistance ratio), and as a consequence the reversal current increases. Accordingly, this measure is also unfavorable. Furthermore, since the in-plane magnetization type arrangement uses magnetic anisotropy that appears in accordance with the shape, the reversal current is sensitive to the variation in shape. Consequently, the variation in reversal current may increase with the advance of downsizing.
By contrast, a so-called perpendicular magnetization magnetic film having the axis of easy magnetization in a direction perpendicular to the film surface can be used as a ferromagnetic material forming the magnetoresistance effect element. When using magnetocrystalline anisotropy in this perpendicular magnetization type arrangement, the element size can be made smaller than that of the in-plane magnetization type arrangement because no shape anisotropy is used. In addition, the dispersion in the direction of easy magnetization can be decreased. Accordingly, the use of a material having high magnetocrystalline anisotropy is expected to achieve both downsizing and a low electric current while maintaining the thermal disturbance resistance.
Examples of a material system for use in the perpendicular magnetization film are an L10 or L11 ordered alloy system (e.g., FePt and CoPt), an artificial lattice system (Co/Pt and Pd), an hcp system (e.g., CoPt), and an RE-TM system (e.g., Tb—CoFe).
The reversal current for reversing magnetization by the spin transfer torque writing method generally depends on saturation magnetization Ms and a magnetic damping constant α of the storage layer. To reverse the magnetization of the storage layer by the spin transfer torque of a low electric current, therefore, it is important to decrease the saturation magnetization Ms and magnetic damping constant α. Also, a device must resist the processing temperature. However, the MTJ is generally formed by a multilayered structure of a plurality of metal films. During an element fabrication process including annealing, therefore, the structure changes in the interface between different types of metals, and the magnetic characteristics readily deteriorate. Accordingly, there is no material that satisfies all of the above-mentioned characteristics necessary for the perpendicular magnetization film as a storage layer.